well for me, MOST of my TTMing is international... I live in Canada and most goes to the States

I order stamps from USPS since I do so much of it in that country...
so that's tip #1 from me, if you plan to do a lot in one country, try to get their postage stamps for your SASE. Canada is probably the best case scenario for this, and mostly only if you collect from hockey players.
For other cases, try to check out what the country's rates are to the States, include that and a few extra bucks for their troubles. I also try to send them money in their own currency. I just go to the bank or local currency exchange and ask for 3 Canadian dollars worth of Czech Korunas for example (which is like 50 Korunas if I remember right). I try to think of it this way... if somebody sent me a few extra American dollars in the mail... I'd be like "well this is a nice gesture, but what am I gonna do with this? I live in Canada"...
Altho I haven't sent much, I've had decent success internationally (not counting the US).
1/2 from Japan
2/2 from Sweden
1/1 from Czech Republic
0/1 from Australia
anyway... that's just me... Good luck with your international TTMing!